1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as power transformers, and more specifically to new and improved winding structures for such apparatus.
2. Description of the Prior Art
Power transformers of the core-form type initially were constructed with circular shaped coils disposed about cruciform shaped legs of a magnetic core. The high cost of vault space in large cities, provided the incentive to develop a more compact power transformer for network applications, and the rectangular core and coil type of construction was developed. The rectangular construction has proven itself to be an extremely rugged, reliable design, as it must be in order to withstand the cable fault surges, switching surges and overload duty characteristics of network operation.
While the rectangular construction is ideally suited to withstand repeated short circuit stresses, means are constantly sought for further improving the strength of the windings to enable them to withstand the tremendous forces applied to the windings during a short circuit condition. During a short circuit applied to the secondary winding of a rectangular core-form power transformer, the outer or high voltage winding is subjected to a force radially outward, and the inner or low voltage winding is subjected to a force directly radially inward.
If it were possible to build a transformer in which the high and low voltage winding or coils were exactly the same length, with uniform and linear distribution of ampere turns per unit of length, and with the ends of the windings in the same plane, the radial forces would be the only significant short circuit forces. In practice, however, taps and manufacturing variations produce a vertical displacement between the electrical centers of the high and low voltage windings, and since the fundamental force of radial repulsion effectively acts between the electrical centers, a force component exists which tends to move the high and low voltage windings in opposite axial directions.
The short circuit force on a winding is proportional to the square of the current in the winding. Short circuit currents may typically be 15 or more times the normal full load current, and the force on the winding is proportional to the square of the current flowing through the winding. The short circuit forces are also increased due to displacement of the first half cycle of current, which displacement is a function of the ratio of the resistance to the reactance of the transformer. The increase in short circuit forces due to this displacement is 3 to 4 times the value of the force with symmetrical current. Thus, the short circuit forces may be 800 to 1000 times the forces existing at normal full load current, and since the axial component is typically in the range of 7 to 15% of the total force, the force which tends to move the windings axially apart during a short circuit is indeed substantial.